In the future mobile communication system, for example, Beyond Third Generation (B3G) or Long Term Evolution-Advanced (LTE-A), the system will provide a higher peak data rate and cell throughput with larger bandwidth. Currently, few un-used frequency band below 2 GHz is left, thus partial or all bandwidth needed by the B3G system can be found only on higher frequency range, 3 GHz or higher for instance. The higher the frequency range is, the quicker the attenuation speed of radio wave propagation becomes, and the shorter the transmission range becomes. Thereby, under the same coverage area, in order to ensure continuous coverage, more eNBs are required. But the high cost of eNBs undoubtedly increases the network deployment cost. For the purpose of reducing the cost and improving coverage, the vendors and standardization organizations start to introduce relay into the cellular communication system.
FIG. 1 is overall LTE-A network architecture with deployed Relay Node (RN), which is wirelessly connected to the core network via the donor cell under a Donor Evolved Node B (DeNB). There is no direct wired interface between the RN and core network and each RN can manage one or more cells. In this architecture, the interface between User Equipment (UE) and RN is called Uu, while that between RN and eNB is Un.
In the architecture, RN has dual roles:
First, RN has a role as a UE. The RN startup procedure is similar to legacy UE attachment procedure. RN has its own Serving-Gateway/PDN-Gateway (S-GW/P-GW) and control plane node Mobility Management Entity (MME);
Second, RN has a role as an eNB for UE. The downlink data of the UE needs to be sent to the serving base station of the UE, i.e. RN, from the SGW/PGW of the UE, and then RN sends the downlink data to the UE via a Uu. According to the discussion in 3rd Generation Partnership Project (3GPP), there are 4 alternative architectures at present.
As shown in FIG. 2, the RN startup procedure is described as (take architecture 1 and 3 for example): radio resource control (RRC) connection is established between the RN and the DeNB; the RN sends Attach Request to the MME; the MME acquires the subscription data of the RN from Home Subscriber Server (HSS) to authenticate the RN; if the authentication is successful, the MME creates default bearer in the S-GW/P-GW for the RN, and then sends an initial UE context setup request message to the DeNB and creates context of RN in DeNB; and then the DeNB sends a RRC connection reconfiguration message to the RN, containing an attach accept message sent to RN by MME; the RN replies RRC connection reconfiguration complete, thus the RN establishes the basic IP connection.
Then, Operation & Maintenance (O&M) downloads the node configuration information to the RN to configure the RN. The RN can work normally like the base station after having established necessary S1 interface and X2 interface.
FIG. 3 shows the RN startup procedure based on architecture 2. Different from the procedure as shown in FIG. 2, in architecture 1 and 3, RN has separate S-GW and P-GW. During RN attach, MME sends create default bearer request to the S-GW/P-GW of RN; in architecture 2, the S-GW and P-GW functions of RN are integrated in the DeNB, so the create default bearer request sent by the MME is sent to DeNB. Besides in architecture 1 and 3, as an eNB, RN needs to establish X2 interfaces with other eNBs and S1 interfaces with MMEs; while in architecture 2 and 4, RN only needs to establish an X2 interface and an S1 interface with the DeNB. To other eNBs and core networks, RN is invisible. The other eNBs and core networks consider the RN as a cell under the DeNB. The DeNB only needs to update the established X2/S1 connection: to add a new cell to other eNBs, and to register a new TAC to the MME if the tracking Area Code (TAC) supported by RN is different from that supported by DeNB.
Currently, scenarios where UE needs to select a cell include: power up, re-entering into the coverage area, RRC connection release and RRC re-establishment. As shown in FIG. 4, a UE, after power up, begins to search Evolved Universal Terrestrial Radio Access (E-UTRA) channel and reports all available Public Land Mobile Networks (PLMN) to non-Access Stratum (NAS), which automatically or manually selects a PLMN and indicate it to an Access Stratum (AS). The UE selects a suitable cell or acceptable cell that meets S criterion on the selected PLMN to camp on, and then the cell reselection procedure starts in order to select a better cell to camp on and avoid unnecessary handover after accessing to the cell. Wherein, two cell selection procedures are available:
(1) Initial Cell selection: This procedure requires no prior knowledge of which RF channels are E-UTRA carriers. The UE scans all the radio frequency channels in E-UTRA bands and finds out at least the strongest cell on each carrier frequency. The UE reads the PLMN of the cell from the system information of the cell. The UE firstly finds a suitable cell, and then an acceptable cell if no suitable cell is found. Once either of them is found, the UE camps on the cell and then the cell reselection starts.
(2) Stored Information Cell Selection: This procedure requires stored information of carrier frequencies and optionally also information on cell parameters, from previously received measurement control information elements or from previously detected cells. But if no suitable cell is found, the initial cell selection starts.
A UE can obtain normal service when camped on a suitable cell. It may obtain limited services when camped on an acceptable cell, such as emergency call.
A UE, after camping on a suitable cell, usually needs to request location registration to the network through attachment procedure, in order to use services requiring registration, such as normal voice call service and WAP. A UE under a restricted status can only make emergency calls.
If a UE finds a more suitable cell in an idle state, it re-selects and camps on such cell.
In the procedure of realizing the objects of the present invention, at least the following problems existing in the current technologies were found:
Though RN has the function of a UE, in one aspect, if RN selects the donor cell according to the current mechanism that a UE selects a cell by S criterion, the selected donor cell may not be expected by the operator; in other aspect, as an eNB, RN needs to enter connected state after start-up as soon as possible in order to serve the UE timely. Unlike a UE, which performs selects cell selection/reselection frequently, an RN needs to select a cell more accurately and rapidly, thus the current cell selection mechanism for a UE is not exactly applicable to an RN to select a suitable donor cell.